Pliocene-Pleistocene lacustrine system in the Eastern Cordillera Basin, Colombia: Paleoenvironmental implications for diatomite deposition

The Plio-Pleistocene Tilata Formation records a tectonic paleo-lake system in the Colombian Andes. This unit exhibits recurrent episodes of volcanism and diatomite deposits. Outcrop-based facies analysis of eight stratigraphic sections varying in thickness from 4 to 40 m indicates variations in the sediment supply, accommodation space, climatic conditions, water depth, and organic activity. Fourteen sedimentary facies are grouped into three facies associations (FAs) representative of alluvial, fluvial, and lacustrine settings. FA1 (alluvial fan deposits) corresponds to conglomerates, sandstones, and subordinated mudstones. FA2 (mixed fluvial system) represents sandy conglomerates, cross-bedded/massive sandstones, organic-rich mudstones, lignites, and diatomaceous mudstones. FA3 (lacustrine) comprises mainly organic-rich mudstones, diatomites, laminated sandstones, and tephra layers. The Upper Tilata Formation reflects the evolution of an overfilled to a balanced-fill lake with a progradational-retrogrational-progradational stacking pattern. The organic facies revealed variations in the origin of the Amorphous Organic Matter (AOM) from aquatic to terrestrial, indicating episodes of shallowing when terrestrial conditions dominated. Seven families, eight genera, and eleven diatom species were identified in diatomites and diatomaceous mudstones, mainly in the lacustrine facies. The most abundant species was Aulacoseira ambigua, suggesting moderate energy in the water column (turbulence), solar light availability for photosynthesis, and mesotrophic to eutrophic conditions. The sedimentological record implies that bedrock, fluvial, pluvial, and volcanic input played a decisive role in nutrient supply for the proliferation of diatoms in the paleo-lake. This scenario is concordant with similar Quaternary depositional systems in northern South America. (c) 2022 Elsevier B.V. All rights reserved.